In conventional radio-frequency signal receivers, the output signal is generally of better quality when the receiver is tuned as close as possible to the carrier or center frequency of the transmitted radio-frequency signal which the operatpr or listener desires to receive. It is well known that when an AM or FM receiver, for example, is tuned to a frequency somewhat removed from the center frequency of the selected station or channel, a significant amount of undesired noise will be present in the audio output of the receiver. As the receiver is tuned to frequencies closer to the center frequency, the audio output becomes less noisy (i.e., the signal-to-noise ratio increases) until the optimum is attained at or very near the center frequency.
In manually-tuned radio-frequency signal receivers, the tuning mechanism may be adjusted until an audio signal is produced which is acceptable to the listener, and a device such as a signal strength meter may be provided to visually aid in the manual tuning thereof. In addition, an automatic frequency control (AFC) circuit may be used to automatically tune the receiver to the center frequency after the receiver has been manually tuned to somewhere within the limits of the control range of the AFC circuit. Another conventional tuning aid is a squelch or muting circuit which, as its name implies, prevents the receiver from producing an audio output signal unless the received signal has a predetermined strength or level.
None of these circuits, however, provides a gating or control signal when the receiver is tuned at or very close to the center frequency of the received radio-frequency signal, and this shortcoming is particularly disadvantageous in a receiver of the scanning type, especially when the selected frequencies of interest are tuned to by means of a digitally-synthesized frequency-generating circuit. This circuit sometimes takes the form of a phase-locked loop (PLL) circuit controlled by a programmable divider.
In a signal-seeking or scanning receiver, a scanning circuit is provided which typically varies the frequency of the local oscillator, either continuously or in preset increments, and a detector circuit is used to stop the scanning operation when a desired signal of adequate strength is received. This detector circuit is often similar to, or part of, a squelch circuit. Conventional detector circuits of this type are made user-adjustable in order to accommodate or compensate for the differences in the strength of the various received signals. Such differences in signal strength can be attributed to a number of factors including differences in distances and orientation between the transmitting antenna and receiving antenna for the various frequencies of interest, as well as differences in the power of the various transmitters. Thus, it is extremely difficult if not impossible to adjust such a circuit so that it will trigger at a preset level of signal strength to provide optimum (i.e., center-frequency) tuning for all signals of interest.
A compromise therefore is often made by broadening the range (e.g., lowering the threshold signal level) of the detector or squelch circuit. This can result in the scanning receiver stopping at a frequency other than the center frequency, especially when the variable tuning is accomplished incrementally as it is in a digital frequency synthesizer circuit. In other words, as the scanner tunes to a strong signal, the threshold signal level for stopping the scanning action will be reached before the scanner tunes to the center frequency at which the signal is strongest. Moreover, if in such a scanner means are provided for displaying the frequency to which the receiver has tuned, the frequency displayed will likewise not be the center frequency.
Accordingly, it is an object of the invention to provide a new and improved system for developing a gating signal when a radio frequency signal receiver is tuned to the center frequency of a selected radio frequency signal.
It is another object of the invention to provide such a new and improved system which is particularly adapted for use in a scanning receiver.
It is a more specific object of the invention to provide such a new and improved system in a scanning receiver of the type which utilizes a digital frequency synthesizer circuit having a digital frequency display of the frequency to which the receiver is tuned.